Toner image transferring type electrophotographic copy machines generally require that a definite toner concentration be maintained for a toner image formed on a photosensitive member in order to regularly maintain a copy image at a definite preferred density. The quantity of toner must therefore be replenished to compensate for the reduction in toner concentration (toner consumption).
On the other hand, the properties of the photosensitive member deteriorate in correspondence with an increase in use. For example, the surface of the photosensitive member may be depleted by contact with the developer and the cleaner blade, and as the thickness of the layer is reduced its chargeability is eroded, thereby reducing the image density. Heretofore, devices have been known, such as that disclosed in Japanese Patent Publication Sho 55-3704, which provide a surface electrometer at the periphery of the photosensitive member to measure the surface potential of said photosensitive member after exposure, and which then compare said measured value to a reference value so as to control the voltage applied to a charging means. However, the electrometers for these devices are expensive, and providing said electrometers in the developing region is structurally impossible. Therefore, the aforesaid devices do not necessarily measure the surface potential in the developing region of the photosensitive member, and have the further disadvantage in that they effectively reduce the image density when the chargeability of the photosensitive member is greatly eroded.